This application claims priorities of Korea patent Application No. 10-2000-0071895, filed on Nov. 30, 2000, and Korea patent Application No. 10-2001-0032822, filed on Jun. 12, 2001.
(a) Field of the Invention
The present invention relates to a shift control method for an automatic transmission of a vehicle, and more particularly, to a shift control method for an automatic transmission for re-setting values of shift-control-variables according to an altitude of the vehicle driving.
(b) Description of the Related Art
Generally, an automatic transmission is exposed to various driving conditions. A transmission control unit determines a most preferable shift-speed under the driving conditions and performs a speed shift to the determined shift-speed.
Whether a speed shift to a target shift-speed is necessary is determined based on a shift-pattern, which includes an upward shift-pattern for determining that an upward shift is necessary and a downward shift-pattern for determining that a downward shift is necessary. As examples of the upward shift-pattern and the downward shift pattern, FIG. 1 shows the upward and downward shift-patterns between a second and a third shift-speed.
Each of the 3xe2x86x922 and 2xe2x86x923 shift-patterns forms a line. When a driving-state determined by a vehicle speed Vs and a throttle valve open-angle TH crosses over the 3xe2x86x922 shift-pattern, a speed shift to the second speed is necessary if the third shift-speed is currently engaged. When a driving-state crosses over the 2xe2x86x923 shift-pattern, a speed shift to the third speed is necessary if the second shift-speed is currently engaged.
Therefore, when the driving-state is to the right of the 2xe2x86x923 shift-pattern, the third shift-speed is engaged, and when the driving-state is to the left of the 3xe2x86x922 shift-pattern, the second shift-speed is engaged. When the driving-state is between the 3xe2x86x922 and 2xe2x86x923 shift-patterns, a target shift-speed is determined based on a hysteresis of the driving-state change.
When the driving state of the vehicle currently corresponds to a point A of FIG. 1, the driving state is changed to a point B if the throttle valve open-angle TH is reduced as an accelerator pedal is released. In this case, a speed shift to the third speed begins if the vehicle is currently running in the second shift-speed. This kind of speed shift, an upward speed shift caused by the throttle valve open-angle being reduced, is called a lift-foot-up shift (referred to as xe2x80x9cLFUxe2x80x9d hereinafter).
To the contrary, when the driving state of the vehicle currently corresponds to a point C, the driving state is changed to a point D if the throttle valve open angle TH is increased as the accelerator pedal is depressed. In this case, a speed shift to the second speed begins if the vehicle is currently running in the third shift-speed. This kind of speed shift, a downward speed shift caused by the throttle valve open-angle being increased, is called a kick-down shift.
The upward shift-pattern is usually modified when the vehicle is running on a slope in order to hold the shift-speed in a lower one such that sufficient engine power can be utilized. Modification factors used for modifying the shift-pattern are determined on the basis of the slope.
FIG. 2A shows an example of the modification factors, and FIG. 2B shows an upward shift-pattern modified under the modification factors.
As shown in FIG. 2A, the modification factors include a modification limit RSU for defining a maximum amount of modification, a minimum slope RS0 for modifying the shift-pattern, and a maximum slope RS1 at which the shift-pattern is modified by the modification limit RSU.
That is, the shift-pattern is not modified when the slope of a road is less than the minimum slope RS0, the shift-pattern is modified by the modification limit RSU when the slope is larger than the maximum slope RS1, and the shift pattern is modified increasingly as the slope increases when the slope is between the minimum and maximum slopes RS0 and RS1.
The solid line 210 of FIG. 2B denotes a 2xe2x86x923 shift-pattern for a level road, and the dotted line 220 denotes a modified 2xe2x86x923 shift-pattern on a slope. When the slope is larger than the minimum slope RS0, the shift-pattern is modified in a rightward direction in FIG. 2B, and the amount of the modification increases as the slope increases.
According to the modified shift-pattern 220, a lift-foot-up shift is prohibited even if the vehicle driving state is changed from the point A to the point B by releasing the accelerator pedal.
A shift control method for a kick-down shift is described hereinafter with an example of 3xe2x86x922 kick-down shift. A friction element for operating in the third shift-speed (referred to as xe2x80x9crelease-elementxe2x80x9d hereinafter) is released during a 3xe2x86x922 kick-down shift-control, and a friction element for operating in the second shift-speed (referred to as xe2x80x9capply-elementxe2x80x9d hereinafter) is controlled to engage during the 3xe2x86x922 kick-down shift-control.
For those operations, hydraulic pressure being supplied to the release-element in the third shift-speed is released and hydraulic pressure is newly supplied to the apply-element, which is realized by controlling solenoid valves included in a hydraulic circuit for controlling hydraulic fluid supply. An example of a duty map of the solenoid valves for releasing the release-element and for engaging the apply-element is shown in FIG. 3.
Lines 310 and 320 respectively represent a release duty and an engaging duty, the release duty denoting the duty for releasing hydraulic pressure of the release-element, the engaging duty denoting the duty for supplying hydraulic pressure to the apply-element.
Control variables for hydraulic pressure release of the release-element include an initial release-duty Dsr for suddenly lowering the duty for hydraulic pressure supplied to the release-element, a ramp-control slope dDr for gradually decreasing the release duty for the release-element from the initial release-duty Dsr, and a high-holding duty Dcr for holding the duty at a higher lever than the duty at an end of the ramp-controlling.
Control variables for hydraulic pressure supply to the apply-element include a low-holding duty Da for holding a low level pressure to reduce an engaging shock of the apply-element, and a transition duty De for supplying sufficient hydraulic pressure to prevent slip of the apply-element.
The density of the air that comes into the engine changes according to the altitude of vehicle driving, which causes changes in engine power. However the prior art does not take the altitude into account in a shift control method for an automatic transmission of a vehicle, especially for a shift control method related to lift-foot-up shift and kick-down shift.
Therefore, there has been a need to provide an effective and appropriate shift control method for appropriately preventing a lift-foot-up shift at a high altitude and for controlling a kick-down shift according to a vehicle driving altitude.
That is, there has been a need to prevent a low atmospheric pressure at a high altitude from causing an operating hydraulic pressure for a speed shift to become relatively high, accordingly to prevent shift shock at high altitude, and to prevent a lift-foot-up shift on a slope at a high altitude, accordingly to have the vehicle provided with sufficient driving force.
The present invention has been made in an effort to solve the above problem. It is an object of the present invention to provide a shift control method for an automatic transmission for preventing a lift-foot-up shift on a slope and for appropriately controlling a kick-down shift when a vehicle equipped with the automatic transmission is driving at a high altitude.
To achieve the above object, the present invention provides a shift control method for an automatic transmission of a vehicle comprising detecting atmospheric pressure, determining whether a predetermined atmospheric pressure applying condition is satisfied, and resetting values of shift control variables based on the detected atmospheric pressure when the predetermined atmospheric pressure applying, condition is satisfied.
The values of shift control variables are reset to be appropriate for an extant atmospheric pressure range, the extant atmospheric pressure range being selected from a plurality of atmospheric pressure ranges formed on the basis of a plurality of predetermined reference atmospheric pressures.
The shift control method of the present invention further comprises determining whether an upward speed shift is necessary based on the reset values of shift control variables.
The shift control variables of which the values are reset comprise at least one of a modification limit RSU for defining a maximum amount of a modification of an upward shift-pattern, a minimum slope RS0 for modifying the upward shift-pattern, and a maximum slope RS1 at which the upward shift pattern is modified by the modification limit RSU.
The shift control method of the present invention further comprises determining whether a predetermined kick-down shift condition is satisfied, and performing a downshift based on the reset values of shift control variables when the kick-down shift condition is satisfied.
The shift control variables of which the values are reset comprise at least one of an initial release-duty Dsr for suddenly lowering a release duty for hydraulic pressure supplied to a release-element, a ramp-control slope dDr for gradually decreasing the release duty for the release-element from the initial release-duty Dsr, and a high-holding duty Dcr for holding duty at a higher lever than a final duty of the ramp-control.
Furthermore, the shift control variables of which the values are reset further comprise at least one of a low-holding duty Da for holding a low level pressure to reduce an engaging shock of an apply-element, and a transition duty De for supplying sufficient hydraulic pressure to prevent slip of the apply-element.
The atmospheric pressure applying condition includes the automatic transmission not undergoing shifting, and the determining whether the predetermined atmospheric pressure applying condition is satisfied determines that the predetermined atmospheric pressure applying condition is not satisfied if the automatic transmission is undergoing shifting.
The atmospheric pressure applying condition preferably includes a change rate of the throttle valve open-angle being within a predetermined change rate range, and the determining whether the predetermined atmospheric pressure applying condition is satisfied determines that the predetermined atmospheric pressure applying condition is not satisfied if the change rate of the throttle valve open-angle is out of the predetermined change rate range.
The atmospheric pressure applying condition further includes the detected atmospheric pressure not being abnormal; and accordingly the determining whether the predetermined atmospheric pressure applying condition is satisfied determines that the predetermined; atmospheric pressure applying condition is satisfied only if the detected atmospheric pressure is not abnormal, and the determining whether the predetermined atmospheric pressure applying condition is satisfied comprises resetting the detected atmospheric pressure as a predetermined atmospheric pressure if the detected atmospheric pressure is abnormal.
The detected atmospheric pressure can be determined to be abnormal when an error exists in a communication line between a transmission control unit and an atmospheric pressure detector, or when the atmospheric pressure detector is determined to be malfunctioning.